DNA methylation is an essential process in growth and development in animals. The importance of DNA methylation in man is underscored by the consequences of its mis-regulation, which include and serious genetics defects and cancer. The process is complicated by the fact that methylation is carried out by a number of different enzymes and their in vivo roles are poorly understood. A genome wide approach to understanding the action and of all catalytically active DNA methyltransferases is proposed using novel technology that allows a snapshot view of the binding event in any given cell in vivo, this method, called the In vivo Complex of Methylase (ICM), will allow empower researchers with the ability to identify genes and cognate DNA binding sites for virtually all catalytically active, endogenous methylases in a chromosomal setting. Because the ICM assay is antibody based, one can readily differentiate methyltransferase isoforms. The approach is highly quantitative and tractable. A series of experiments are proposed, based on current tools of biochemistry and molecular biology of DNA methylation reaction mechanism, to develop products that allow researchers insight into specific methylation activities in cancer. These products will be provided to the scientific community at large for use in cancer research. The goal is to empower researchers with the ability to quantify methylase: genome interactions in living cells. Given the acute interest of methylation as a molecular target in cancer, these experiments are also relevant to the development of effective anti-cancer therapy using DNA hypomethylating agents. Finally, the ICM method represents a clearly defined and novel biological correlate that could be a significant benefit in clinical trials with DNA hypomethylating drugs.